LIN7 Cell-Wall Invertase Orthologs in Cultivated and Wild Tomatoes (Solanum Section Lycopersicon)

Plant acid invertases are considered to be the key enzymes in sucrose unloading and carbohydrate supply to sink tissues. Acid cell-wall invertases control sucrose transport via the apoplastic pathway during sink initiation and expansion. In this study, we identified 12 LIN7 gene homologs encoding cell-wall invertases in red- and green-fruited tomato accessions (Solanum section Lycopersicon) of self-compatible and self-incompatible species. All genes consisted of six exons and five introns, including highly conserved 9-bp exon II. Identification of 226 exonic single nucleotide polymorphisms as well as extremely high intron variability indicates a significant interspecific divergence among the examined tomato accessions. Computational prediction revealed protein structure typical for the glycosyl hydrolase family 32 and conserved catalytic sites described for other plant cell-wall invertases. LIN7 expression in mature buds and flowers confirms LIN7 role in the development of pollen tubes and grains. The variability in gene and protein sequences and species-specific differences in LIN7 expression patterns may be responsible for putative functional divergence of invertases. Furthermore, we performed phylogenetic analysis of the Solanum section Lycopersicon species based on the LIN7 gene, which clearly divided the analyzed tomato accessions into two main clusters corresponding to self-compatible and self-incompatible species and was in agreement with the separation into red- and green-fruited plants. Given that LIN7 plays an essential role in tomato fertility and fruit ripening, the characterization of protein variability within species of section Lycopersicon may be useful to evaluate the potential application of the encoding genes for tomato breeding programs.